Habits can exert powerful control over behavior. Sometimes habits are maladaptive, such as in substance abuse. In many cases, however, habits can be adaptive, enabling organisms to engage in healthy choices rapidly and efficiently. In recent years there has been substantial progress in understanding the neural mechanisms that support the transition from effortful, goal-directed to more automatic, habitual behavior through research in rodents. This work has emphasized the role of dopamine and its striatal targets in habits and has begun to suggest the principles by which their strength is controlled. Surprisingly, however, the rodent research has led to only limited progress in understanding the balance between goal-directed vs. habitual behaviors in humans, preventing the scientific community from being able to apply this knowledge to clinical settings and everyday life. We hypothesize that the main reason for this gap is the lack of rich and precise behavioral markers for habitual behavior in laboratory settings in humans. The proposed research program aims to address this gap. We propose a series of functional imaging (fMRI) and neuropsychological studies that focus on computational characterization of distinct types of learning and the factors that impact their relative strength. Because both habits and goal-directed behaviors depend on past experience, we can leverage recent advances in characterizing brain systems for learning and memory in humans that have already led to a rich, quantitative characterization of multiple aspects of behavior and neural signaling. Using these graded and dynamic signatures of habit formation - specifically, by examining choices and choice-related fMRI signals during trial-and-error learning, and during subsequent probes of the memories formed - we propose to ask a series of questions collectively aimed at uncovering the mechanisms by which the brain balances goal-directed and habitual behaviors. Our specific aims are: (1) To determine how the capacity to form flexible memory representations modulates goal-directed and habitual systems (2) To understand how the timing of feedback modulates the balance between the systems;(3) To determine how the reliability of feedback modulates goal-directed and habitual systems. For each aim, we test healthy subjects with fMRI to determine the dynamic contribution of multiple brain regions to different aspects of behavior. Parallel studies with Parkinson's patients complement the fMRI studies and provide evidence about the causal role of dopaminergic inputs to the striatum in habit learning. Results from this research will advance understanding of the behavioral and neurobiological mechanisms that control the emergence of habits and the implications for decisions and actions. Determining how multiple brain systems for learning support the transition of behavior from goal-directed to habitual control will lay the foundation for future translational work on potential treatment interventions that can adaptively shift behavior towards the formation of healthy habits in both health and disease. PUBLIC HEALTH RELEVANCE: Habits can be highly adaptive, leading us to choose healthy foods, preventing us from forgetting to take our medication and helping us exercise routinely without having to invest unnecessary energy in repeatedly deliberating between multiple options. We propose an interdisciplinary research program to investigate what habits are, how they are formed, and what are the factors that control their emergence. These studies will advance understanding of the neurobiological, computational and cognitive factors underlying habit formation and may help us discover how best to modulate behavior so as to promote healthy habits across multiple domains.